A MIMO system employs multiple (NT) transmit antennas at a transmitter and multiple (NR) receive antennas at a receiver for data transmission. A MIMO channel formed by the NT transmit antennas and NR receive antennas may be decomposed into NS spatial channels, where NS≦min{NT, NR}. The NS spatial channels may be used to transmit data in parallel to achieve higher throughput and/or redundantly to achieve greater reliability.
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation scheme that effectively partitions the overall system bandwidth into multiple (NF) orthogonal subbands. These subbands are also referred to as tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data.
For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), NF subbands are available on each of the NS spatial channels for data transmission. The NF subbands of each spatial channel may experience different channel conditions (e.g., different fading, multipath, and interference effects) and may achieve different channel gains and signal-to-noise-and-interference ratios (SNRs). Depending on the multipath profile of the MIMO channel, the channel gains and SNRs may vary widely across the NF subbands of each spatial channel and may further vary widely among the NS spatial channels.
For the MIMO-OFDM system, one modulation symbol may be transmitted on each subband of each spatial channel, and up to NF·NS modulation symbols may be transmitted simultaneously in each OFDM symbol period. Each transmitted modulation symbol is distorted by the channel gain for the subband of the spatial channel via which the symbol is transmitted and further degraded by channel noise and interference. For a multipath MIMO channel, which is a MIMO channel with a frequency response that is not flat, the number of information bits that may be reliably transmitted on each subband of each spatial channel may vary from subband to subband and from spatial channel to spatial channel. The different transmission capabilities of the different subbands and spatial channels plus the time-variant nature of the MIMO channel make it challenging to ascertain the true transmission capacity of the MIMO-OFDM system.
There is therefore a need in the art for techniques to accurately determine the transmission capacity of the MIMO-OFDM system for efficient data transmission.